Equation 33.3 the substance from blood into tubule fluid. The first step in P × C = U × V˜ urine formation by the kidneys is production of an ultrafil-xa x x trate of plasma across the glomerulus. The process of glo-If
Eq. 33.3 is rearranged and the concentration of merular filtration and regulation of GFR and RBF substance x in renal artery plasma ( ) is assumed to beare discussed later in this chapter. The concept of renal clearance, which is the theoretical basis for measurement of GFR and RBF, is presented in the following section. Reabsorption and secretion are discussed in subsequent chapters.
The concept of renal clearance is based on the Fick principle (i.e., mass balance or conservation of mass).
Fig.
33.12 illustrates the various factors required to describe the mass balance relationships of a kidney. The renal artery is the single input source to the kidney for substances not synthesized by this organ, whereas the renal vein and ureter constitute the two principal output routes. In other words a nonmetabolized substance entering the renal circulation via the renal artery may only exit this circulation via the renal vein (i.e., the unfiltered fraction plus any filtered amount subsequently reabsorbed back into blood) or the ureter (the combined filtered and secreted fractions less any tubular reabsorption). The following equation defines the mass balance relationship:
Equation 33.1 where identical to its concentration in a plasma sample from any peripheral blood vessel (Px), the following relationship is obtained:
Equation 33.4
Clearance has the dimensions of volume/time, and it represents a volume of plasma from which all the substance has been removed and excreted into urine per unit time. This last point is best illustrated by considering the following example. If a substance is present in urine at a concentration of 100 mg/mL and the urine flow rate is 1 mL/min, the excretion rate for this substance is calculated as follows:
Equation 33.5
If this substance is present in plasma at a concentration of 1 mg/mL, its clearance according to
Eq. 33.4 is as follows:
Equation 33.6 are the concentrations of substance x in the renal artery and renal vein plasma, respectively
RPFa and RPFv are renal plasma flow rates in the artery and vein, respectively In other words, 100 mL of plasma will be completely Ux is the concentration of substance x in urine cleared of substance x each minute. The definition of clearV̇ is the urine flow rate ance as a volume of plasma from which all the substance . UCr x V
Amount filtered PCr x GFR = Amount excreted UCr x V .
• Fig. 33.13 Renal handling of creatinine. Creatinine is freely filtered across the glomerulus and is, to a first approximation, not reabsorbed, secreted, or metabolized by the nephron. Note that all the creatinine coming to the kidney in the renal artery does not get filtered at the glomerulus (normally, 15%–20% of plasma creatinine is filtered). The portion that is not filtered is returned to the systemic circulation in the renal vein. PCr, plasma creatinine concentration; RPF, renal plasma flow; Ucr, urinary concentration of creatinine; V., urine flow rate.
has been removed and excreted into urine per unit time is somewhat misleading because it is not a real volume of plasma; rather it is a virtual volume.
The concept of clearance is important because it can be used to measure GFR and RPF and determine whether a substance is reabsorbed or secreted along the nephron.
The GFR is equal to the sum of the filtration rates of all functioning nephrons. Thus it is an aggregate index of kidney function. A fall in GFR generally means the kidney disease is progressing, whereas recovery generally suggests recuperation. Thus serial assessment of a patient’s GFR is essential to evaluate the severity and course of kidney disease.